Steel alloy



Patented Jan. '11, 1938 STEEL ALLOY William F. Finkl, Chicago, 11].,assignor to A.

Finkl & Sons Company, Chicago, 111., a corporation of Illinois NoDrawing. Application March 4, 1936.

Serial No. 67,069

13 Claims.

This invention relates to steel alloys which are particularly adaptedfor use in pieces of relatively large or heavy sections such as dieblocks used for hot work forging processes, and other steel lo thetendency to develop internal thermal ruptures or cracks is practicallyeliminated, and the resulting piece is capable of being hardened to asubstantially uniform hardness throughout its mass by proper heattreatment.

The present invention is an improvement over my prior United StatesLetters Patent No. 1,464,174 issued August 7, 1923, in which theprincipal alloying elements are chromium, nickel and molybdenum. Alloysteels made in accordance with this patent have demonstrated theirsuperiority for hot work dies and similar forging work. At present,approximately ninety per cent of the dies used in the drop forgingindustry are made from chromium-nickel-molybdenum steel 35 alloy in oneor another of its various forms.

The present application is ,a continuation in part of my priorapplications Serial No. 33,331 filed July'26, 1935, and Serial No.35,104, filed August '7, 1935.

30 Among the many methods of mass production used in modern industry,there are none more important than that of drop forging and its nearrelation, upsetting. -By these two processes, which may both be referredto generally by the accurately metal parts of surprisingly, intricatedesign in large quantities with remarkable speed.

The method in general consists of heating the metal (usually steel) to aplastic condition and forming it in dies which contain the desired shapein negative. The dies are operated by powerful hammers in the case ofdrop forging, and by forging machines in the case of upsetting.

In order that the drop forger may do his job well and profitably, hisdies must produce thousands of duplicate parts without any great wear ordeterioration. This'calls for a die steel having great strength anddurability. since the dies are subjected to extremely heavy shocks andhigh temperatures. The dies are usually furnished to the drop forger inthe form of blocks in either the annealed or tempered condition. Ifannealed, the dies must be hardened and tempered after being, machined.If tempered, the dies are used in the hammer immediately after machiningwithout further heat treatment.

Over 60% of the dies used in the industry today are thus tempered. Theblock is heattreated in the rough condition by the die block 5manufacturer, then machined by the die maker, and then put to use by theforger without further heat-treatment. After the initial die impressionis worn out from use, it is machined off the die block, and a newimpression remachined in the fresh metal surface of the block, and theblock is used again without further heat-treatment. This remachiningoperation may be repeated several times, often as many as seven times,until the blockbecomes too thin for economical use.

One of the principal difliculties encountered by the manufacturers ofalloy steels of the nickelchromium-molybdenum class is that of avoidingthermal ruptures when the alloy is first produced at the mill. Thermalrupture (also known as flaking) should not be confused with "centerporosity" which occurs in a wide variety of metals, in the form of smallvoids and segregations. Thermal rupture consists in the formation ofminute cracks which develop on the interior of the heavy mass of metal,when it is cooled from its originalsolidification temperatures. Asthesecracks or flakes are entirely confined to the interior of the block,their presence is practically impossible of detection without cuttinginto the body of theblock, and even then they sometimes cannot bevisually detected unless the exposed surface is etched to show itsgrain. Hence such ruptures will not usually become apparent until afterthe block has been rough-machined, and perhaps given its final heattreatment, The defect may not even be discovered when the firstimpression or die is cut, but such defects soon show up when in thehammer, and the particular piece is rendered useless as a die-block. Ininstances where such alloys are used for heavy structural purposes, thedefect usually does not become. evident until an unexpected failure orbreakage occurs.

The exact cause of thermal ruptures or flaking does not seem to havebeendefinitely ascertained, and heretofore the only practical method ofcontrol has been through great care in the manufacture of alloy steelsof this general character 50 whereby cooling of the massive sectionsfrom their original rolling or forging temperatures is carefullyretarded in cooling pits by various methods well known to those skilledin the art. But

'even such great care in cooling has provedin- 55 sufllcient toeliminate frequent recurrence: of

. Per cent Carbon .25 to 1.10 Chromium .25 to 1.50 Molybdenum .05 to2.00 Copper .25 to 6.50

Such alloys may also have nickel in amounts up to 3.00%, manganese inamounts up to 1.00%, and silicon in-amounts up to 1.00%. In alloy steelscoming within the above ranges having copper added in the amountsindicated, the tendency to thermal ruptures or flaking can beeffectively controlled so as to greatly reduce the care in handlingthese steels during their manufacture, and thereby insin'ing practicalfreedom from such internal defects.

Another desirable characteristic heretofore I unattained in steel alloysof the types generally used for die blocks, is that of maintainingsubstantially the same hardness throughout the entire mass of a dieblock so as to permit several successive "sinkings of dies therein,without requiring re-heat treatments, and without reducing theeffectiveness of each such successive die sinking. In steel die blocksheretofore employed, there is usuallya substantial reduction in hardnesstoward the center of the block, with a resulting loss in production fromsuch softer die faces, and a greater tendency to cracking or breakage ofthe die due to the variation in hardness from the outside edge to themiddle of such center sinkings.

I findthat the'addition of copper in certain proportions to alloy steelsused in die blocks or similar heavy-sectioned masses gives "asubstantially uniform depth of hardnesses at die-working values, whichuniformity of hardness has hitherto been considered unattainable.further that the addition of copper appreciably increases resistance tochecking or surface cracking from heat, which are also common faults ofalloy die steels used in hot work.

Summarizing the ideal requirements for a hot work die steel, it musthave the following properties to a marked degree:

l. Resistance to internal thermal crackingduring cooling from hightemperatures.

2. Complete hardenability. in large sections (the ability to harden withcomplete pniformity -of hardness throughout the mass of the piece).

3. Resistance to heat cracking (surface crackingduetoheat).

4. Resistance to washing (localized wear). l 5. Ease and simplicity ofmanufacture in either the open hearth or electric furnace processes toproduce sound'ingots and billets with practically the total absence ofsegregation, center porosity, and hot shortness.

6. Easy forgeability into the necessary size and shape.

7. Simple heat treatment to required 'hardness.

8. Great strength and density of fibre combined with high ductibilityand high values.

9. Machinability at high (working) hardnesses.

The importance of properties Nos. 1 and 2 has already been set; forth.Alloy steels heretofore employed in heavy sections for die blockpurposes are especially subject to internal rupture cracks or flaking.Property No. 2 is most'important in a tempered die block, whereassuccessive recuts or sinkings are made, the center of the block isapproached and used. In most impact die steels that have been hardened,the hardness w drops off toward the center of the block.

As to properties Nos. 3 and 4, these are opposing characteristics. Whenthe die is too soft it will wash, when too hard it may heat-check. Acompromise hardness where these opposite faults are at a minimum isusually necessary.-

Obviously a steel that will resist heat-checking at any and allhardnesses is the ideal, since washing may be overcome by simply raisingthe hardness sufliciently.

Property No. 5.-In the .large sizes in which the'modern die block ismade, it is important that the total alloy content be low, since thehigher this content goes the more diflicult it is to produce soundingots in such large sizes.

Property No. 6.It is obvious that to produce die blocks commercially,the steel must be readil y forged. This is another reason for low alloycontent. In general, the higher the alloy content, the greater are thedifficulties in forging.

Property No. 7.-Heat treating processes are expensive in their simplestforms, and therefore complicated treatments are to be avoided.

Property No. 8.Die blocks must undergo ex- .treme. conditions of shock,heat and abrasion, a

combination of opposing conditions very difl'icult to meet.

Property No. 9.In the tempered die block it is necessary to leave thesteel atthe relatively high hardness suitable for use as a die, 'yet itmust be machinable at this high'hardness.

The nickebchromium-molybdenum die steels heretofore used possess theproperty numbered above 5 to 9, both' inclusive, to'a high degree. Othertypes of steels, of course, have been developed having one or'more ofthese characteristics. I have demonstrated, however, that the alloyof,my present invention combines all of the qualities to an unusualdegree never found in any other steels. This is particularly so with thepropertiesof resistance to internal thermal ruptures, and complete oruniform hardness at relatively high working hardnesses. X

Referring now' more' specifically to the analysis of die block steels inwhich substantial uniformity of hardness at high working hardnesses isdesired, together with the other desirable properties of such steelshereinbefore listed, I find that theranges of the principal alloyingelements should be restricted, substantially as follows:

v I Per cent Carbon .30 to 1.00 phromium nun .25 to 1.25 Molybdenum .05to 2.00 Nickel .50 to 2.50 Copper .25 to 4.00

Manganese in fractions to 1.00% and silicon in fractions to 1.00% mayalso be present, the remainder being substantially all iron.

- I find that the addition of certain other e1ements commonly used inspecial alloys such as tungsten, cobalt, vanadium, or titanium seem tohave no appreciable effect in improving the alloy Per cent Carbon .30 to.70 Nickel .50 to 2.50 Chromium. .25 to 1.25 Molybdenum .05 to 2.00Copper .50 to 3.50

The ranges above specified are critical to the economical production ofa hot work die steel having the nine necessary characteristics-mentionedabove toan exceptional degree. Of particular importance is itsremarkable resistance to thermal rupture, or flaking, its complete oruniform hardenability at working hardnesses, and its resistance toheat-checking.

I have yet to encounter thermal ruptures or flakes in the many melts ofthis improved alloy so far prepared, even though in some instancesdeliberate attempts have been made to produce them. This characteristic,therefore, permits less expensive methods of handling during fabricationfrom the ingot to the billet, and ultimately to the die block.- It alsoshortens the time required for cooling the steel, andthus saves in thePer cent Carbon .50 Manganese .75 Chromium .75 Copper 2.00 Nickel 1.00Molybdenum .25

The block was cut through the middle, and hardness readings taken on thecut surface at intervals from the center to all sides thereof. It wasfound that the hardness was uniform throughout the surface within atotal range of .05 mm. Brinell ball diameter at 350 Brinell hardnessnumber. This substantial uniformity is remarkable; greater uniformitythan this could hardly be expected, even over the outside surface of theblock.

The improvedresistance to heat checking noted in the alloy makes itpossible to employ a higher hardness than normal, thus improvingresistance to washing. In those applications to dies in which this alloyhas been used, heat checking has been practically unknown.

The presence of copper seems to be largely responsible for the importantproperties of resistance to thermal rupture and complete hardenability,as well as increasing the air hardening efiect heretofore noted innickel-chromium-mo lybdenum alloys and raising their tensile properties.Nickel of 50% or more is necessary to contribute the required toughness.Nickel also helps to overcome the tendency for copper-bearing steels toroughen at the surface or to become hot short during forging or rolling,but I find that the addition of molybdenum in small quantities seems tointensify this action of nickel to a marked degree, thereby reducing theamount of nickel necessary in alloys of this character,

and also making it possible to use more copper with its attendantadvantages. Over 2.50% nickel makes the steel difficult to fabricate andadds greatly to the expense of the alloy.

The air hardening qualities of my improved alloy appear to followsubstantially the same laws disclosed in my prior Patent No. 1,464,174

in so far as variation in the principal alloying' elements is concerned.That is to say, superior air hardening qualities may be maintained byvarying the carbon content inversely with the chromium or molybdenumcontent, and the amount of chromium and molybdenum may also be variedinversely with each other within the ranges indicated. In the presentimproved alloy, however, copper can be used to produce a more economicalproduct by partially replacing both the nickel and molybdenum, and withthe new and added results of resistance to thermal rupture, and greatlyincreased uniformity of hardening.

My alloy can be annealed by heating to above the critical range andcooling slowly in the furnace. Hardening is accomplished by heating toabove the critical range and cooling in still air, in an air blast,or'by quenching in a liquid medium. The more rapid the cooling method,the

harder will be the steel. The steel may be tem-- pared to the desiredpoint after hardening by the critical range. ing ranges, the hardnesswill be substantially uniform throughout the mass of the piece.

I claim as my invention:

1. A die block for hot forging characterized by its resistance tothermal rupture and substantial uniformity of hardness at workinghardnesses, which comprises as its principal alloying elements carbonranging from .30 to 1.00%, chromium from .25 to 1.25%, molybdenum from.05 to 2.00%, nickel from .50 to 2.50%, and copper from .25 to 4.00%,the remainder being substantially all iron.

2. A die block for hot forging characterized by its resistance tothermal rupture and heat checking, and its substantial uniformity ofhardness at working hardnesses, which comprises as its principalalloying elements carbon ranging from .30 to 170%, nickel from .50 to2.50%, chromium from'.25 to 1.25 molybdenum from .05 to 2.00%.

and copper from .50 to 3.50%, and the balance 5 to thermal rupture, andby substantial uniformity of depth hardening, and containing carbonranging from .25 to 1.10%, nickel from .50 to 3.00%, chromium from .25to 1.50%, molybdenum from .05 to 2.00%, copper from .25 to 6.50%, andthe balance substantially all iron.

4. A steel alloy characterized by itsresistance to thermal rupture, andby substantial uniformity of depth hardening, and containing carbonranging from .30 to 1.00%, nickel from .50 to 2.50%, chromium from .25to 1.25%, molybdenum from .05 to 2.00%, copper from .25 to 4.00%, andthe balance substantially all iron.

'5. A steel alloy characterized by its resistance to thermal rupture,and by substantial uniformity of depth hardening, and containingcarbonfrom .30 to 170%, nickel from .50 to2.50%, molybdenum from .05 to 2.00%,chromium from .25 75 1.25%, copper from .50 to 3.50%, and the balancesubstantially all iron. 4

6. A steel alloy characterized by its resistance to thermal rupture, andby substantial unlform- I for hot forging made of steel alloycharacterized by'substantial uniformity of depth hardening and itspractical freedom from thermal rupture, said steel alloy containingcarbon ranging from .25 to 1.10%, nickel ranging from .50 to 3.00%,chro-, mium ranging from .25 to 1.50%, molybdenum ranging'from .05 to2.00%, copper ranging from .25 to 6.50%, and thebalance substantiallyall iron.

8. A steel alloy characterized by its resistance to thermal rupture andby substantial uniformity of depth hardening, and containing carbonranging from .30 to 1.00%, nickel from .50 to 2.50%, chromium from .25to 1.25% molybdenum from .05 to 2.00%, copper from .25 to 4.00%, and thebalance substantially all iron, the molybdenum content varyingapproximately in inverse proportions to the carbon content.

9. A steel alloy characterized by its resistance to thermal rupture andby substantial uniformity of depth hardening, and containing carbonranging from .30 to 370%, nickel from -.50 to 2.50%, chromium from .25to 1.25%, molybdenum from .05 to 2.00%, copper from .50 to 3.50%, andthe balance substantially all iron, the molybdenum content varyingapproximately in inverse proportionsto the carbon content.

10. A steel alloy characterized by its resistance to thermal rupture andby substantial uniform ity of depth hardening, and containing carbonranging 'from..25 to 1.10%, nickel from .50 to 3.00%, chromium from .25to 1.50%, molybdenum from .05 to 2.00%, copper from .25 to 6.00%, andthe balance substantially all iron, the molybdenumand chromium contents,respectively,

varying in approximately inverse proportions to the carbon content.

'11 A steel alloy characterized by its resistance to thermal rupture andby substantial uniformity of depth hardening, and containing carbonranging from .30 to 370%, nickel from .50 to 2.50%, chromium from .25 to1.00%, molybdenum from .05 to 2.00%, copper from .50 to 3.50%, and thebalance substantially all iron, the molyba denum and chromium contents,respectively, varying in approximately inverse proportions to the carboncontent.

12. A new article of manufacture, a tempered die block for hot forgingmade of steel alloy containing carbon ranging from .25 to 1.10%, nickelfrom .50 to 2.50%, chromium from .25 to 1.50%, molybdenum from .05 to2.00%, copper from .25 to 6.50%, and the balance substantially all iron.

13. A new article of manufacture, a'tempered die block for hot forgingmade of steel alloy containing carbon ranging from .30 to .70%, nickelfrom .50 to 2.50%, chromium from .25 to 1.25%,

molybdenum from .05 to 2.00%, and copper from V

